The invention relates to an arrangement for measuring the level of contents in a container.
When measuring contents levels a difference is made between devices measuring the contents level continuously, which can give a measured value at all times of the current contents level in the container, and limit sensors, which only display whether the contents level in the container is above or below a predefined level to be monitored by the limit sensor. Such limit sensors are especially used for monitoring a maximum or minimum contents level in order to prevent overfilling or underfilling of the container. Different types of limit sensors are known, for example capacitive or resistive limit sensors, which respond to the electrical properties of the material in the container, limit sensors with oscillating elements, such as membranes or vibrating rods, whose vibrations are damped by the material in the container or whose self-resonant frequency is modified by the material in the container, etc. The selection of the limit sensor used depends on the properties of the material in the container and on the application conditions. In each case, a limit sensor gives an electrical signal which has a first signal value when the contents level is below the level to be monitored, and which has a different second signal value when the contents level is above the level to be monitored. The signal changes its signal value at exactly the moment the contents level passes the level to be monitored through addition of material to or removal of material from the container. With several types of limit sensors the signal value depends on whether the limit sensor is in contact with the material in the container or not; with such limit sensors the predefined level of contents monitored by the limit sensor corresponds to the level of installation of the limit sensor.
DE 39 04 824 A1 discloses a level measuring arrangement which on the one hand has a mechanical level measuring device, e.g. with a float-operated transmission linkage, for the continuous measurement of the contents level, and on the other hand an electrical limit sensor for limiting the maximum permissible contents level. When the contents have reached the maximum permissible level, the limit sensor gives an electrical signal which triggers an electronically controlled automatic level limiter irrespective of the contents level indicated by the mechanical level measuring device, and for example interrupts the filling of the container. In this way, overfilling of the container is prevented also in case of a defect or inaccuracy in the mechanical contents level measuring device.
On the other hand, continuously measuring contents level measuring devices which are distance meters working according to the transit time principle are also known. Thus a device is disclosed in U.S. Pat. No. 5,309,763 for continuous measurement of the level of a liquid in a container; this device has a tube which extends upwardly from the bottom of the container and which is filled with liquid, whereby the level inside the tube is identical with the level outside the tube. A number of reflectors are mounted along the length of the tube. At the lower end of the tube is a transceiver unit which emits ultrasonic pulses upwards through the liquid in the tube and receives the ultrasonic pulses reflected from the submerged reflectors and from the liquid surface. A measuring circuit calculates the level of the liquid from the difference between the arrival times of the ultrasonic pulses reflected from the two uppermost submerged reflectors and the difference between the arrival times of the ultrasonic pulses reflected from the surface of the liquid and the uppermost submerged reflector. In this way, measurement is independent of changes in the acoustic properties within the liquid, in particular material and temperature-related changes in the sonic velocity. The use of a tube on the one hand results in a concentration of the ultrasonic waves on a small area of the liquid surface, and on the other hand prevents the influence of interfering reflections and external sources of interference.
A level measuring device disclosed in U.S. Pat. No. 5,095,748 is designed very similarly, but with the difference that two parallel tubes are provided, one of which contains the reflectors, while the other tube is used for the measurement of the transit time to the liquid surface; each of the two tubes is equipped with its own transceiver unit for ultrasonic waves.
A precondition for this prior art technique is that a tube is fixed in the container extending more or less over the whole height of the container. This represents no problem in small containers, such as fuel tanks in vehicles or airplanes, but often proves to be difficult or impossible in large containers. Another precondition for this prior art technique is that the transceiver units for the waves used for the transit time measurement are located at the lower end of the tube and are thus exposed to the liquid in the container, which is not possible with aggressive or very hot materials. A final precondition for this prior art technique is that the waves are transmitted through the liquid, which in many cases is only possible with the use of ultrasonic waves, but not with the use of very short electromagnetic waves (microwaves), such as are increasingly used for level measurement according to the transit time principle.
The difficulties arising in the technique described above are avoided in a level measuring device disclosed, for example, in U.S. Pat. No. 4,972,386, which also works according to the transit time principle, but where the waves are directed downwards from a position above the highest level of the material in the container onto the surface of the material in the container. In this case the transmitted waves as well as the reflected echo waves do not go through the material in the container, but through the air located above the material in the container, and the transceiver unit is not in direct contact with the material in the container. The waves used to measure the transit time can be microwaves or ultrasonic waves. In both cases the distance between the level measuring device and surface of the material in the container results from the transit time of the waves corresponding to the useful echo reflected from the surface of the material in the container. The contents level can be calculated directly from this distance.
With this method, a precondition for correct contents level measuring is that the useful echo is unequivocally identified from the total of the echo waves received. For this purpose it is usual to form an echo function from the echo waves received representing the amplitudes as a function of distance. Under ideal conditions, this echo function shows an absolute maximum, the position of which represents the useful echo and thus the desired distance between the level measuring device and the surface of the material in the container. In practice, however, interference occurs, and this makes the evaluation of the echo function difficult or even impossible. First of all, the noise background in the close range is considerably higher. With ultrasonic units this is due to the dying out of the electro-acoustical transducer, and with microwave units it is due to multiple reflections in the area of the signal input and of the antenna. Furthermore, echoes are produced not only on the surface of the material in the container but also on other reflecting structures in the beam path. Particularly with level measurement in containers, significant interfering echoes occur as a result of reflections from container walls, weld seams and built-in components such as tubes, heating elements, limit sensors etc., and the useful echo must be distinguished from these. In particular it is not possible to assume that the useful echo is identical with the absolute maximum of the echo function.
It is an object of the invention to permit automatic checking of the measured result and, if necessary, the elimination of a measuring error in an arrangement suitable for continuous measurement of the contents level in a container in accordance with the transit time principle, irrespective of the size of the container and the type and properties of the material in the container.
The invention offers a solution to this problem by providing an arrangement for measuring the contents level in a container
comprising a level measuring device for continuously measuring the contents level and which is a distance meter functioning in accordance with the transit time principle,
which has a contents level sensor which transmits waves from a position located above the highest possible contents level to the surface of the material in the container, receives the echo waves reflected from the surface of the material in the container, and delivers an electric output signal representing the echo waves,
and which has a measuring circuit which determines the useful echo reflected from the surface of the material in the container from the output signal of the contents level sensor and determines the measured value of the contents level from the transit time of the useful echo,
and comprising at least one limit sensor located at the container to supply an electrical signal indicating whether the contents level in the container is greater or smaller than a contents level to be monitored by the limit sensor and which is located between the minimum and maximum contents level of the container,
the measuring circuit receiving the output signal of the or of each limit sensor, and using this to check the measured contents level value determined by the level measuring device.
With the arrangement according to the invention, a continuous plausibility check of the measuring result of the continuous contents level measurement can be carried out on the basis of the contents level displays of one or of several limit sensors, and this check is the more accurate the greater the number of limit sensors used is. Furthermore, each time the contents level in the container changes, each limit sensor gives an exact contents level measured value when the contents level is identical with the level to be monitored by the limit sensor. A comparison of this measured value with the measured value supplied simultaneously by the level measuring device can show whether there has been a measuring error, and when a predefined error limit is exceeded, measures can be initiated to eliminate the measuring error, or a warning or error message can be displayed. This allows the field of application of continuously measuring level measuring devices to be extended and the measuring reliability to be increased under difficult application conditions.
A first embodiment of the invention consists in that several limit sensors are arranged at the container to monitor different levels of contents, and at every status change of the output signal of a limit sensor occurring when the contents level reaches the level to be monitored by this limit sensor, the measuring circuit compares this level with the contents level measured value supplied at the same time by the level measuring device in order to check the correctness of the measurement carried out by the level measuring device.
When the measuring circuit forms an echo function from the output signal of the contents level sensor representing the echo amplitudes as a function of the distance over the whole measuring range, and compares each current echo function with a stored empty echo function recorded with an empty container in order to determine the useful echo corresponding to the echo waves reflected from the surface of the material in the container, then the arrangement is preferably designed in such a way that each time the level comparison results in an error exceeding a given error limit, the measuring circuit actualizes the empty echo function from above to the level which is to be monitored by the limit sensor whose output signal has changed its status.
In this first embodiment it is also possible to make provision that every time the output signal of a limit sensor changes its status the measuring circuit actualizes the stored empty echo function from above to the level to be monitored by this limit sensor, irrespective of the result of the level comparison.
A further development of this first embodiment consists in that the measuring circuit actualizes the stored empty echo function at predefined intervals in the range of the limit sensors whose output signals indicate that the contents level is below the levels to be monitored by them.
Furthermore it is possible to make provision that for determining the useful echo the measuring circuit evaluates only that part of the current echo function situated in the range between the levels to be monitored by two limit sensors, one level sensor of which indicates that the contents level is above the level to be monitored by it, and the other level sensor of which indicates that the contents level is below the level to be monitored by it.
Finally, an advantageous modification of this first embodiment consists in that the measuring circuit checks to see whether the measured level value supplied by the level measuring device is situated in the range between the levels to be monitored by two limit sensors, one level sensor of which gives a signal indicating that the contents level is above the level to be monitored by it, and the other level sensor of which gives a signal indicating that the contents level is below the level to be monitored by it.
A second embodiment of the invention consists in that one limit sensor is located at the container, and at each change in status of the output signal of the limit sensor occurring when the contents level reaches the level to be monitored by the limit sensor, the measuring circuit compares this level with the contents level measured value simultaneously supplied by the level measuring device, in order to check the correctness of the measurement carried out by the level measuring device.
When the measuring circuit forms an echo function from the output signal of the contents level sensor representing the echo amplitudes as a function of the distance over the entire measuring range, and compares each current echo function with a stored empty echo function recorded with an empty container in order to determine the useful echo corresponding to the echo waves reflected from the surface of the material in the container, then the arrangement is preferably designed in such a way that the measuring circuit actualizes the empty echo function from above to the level to be monitored by the limit sensor every time the level comparison results in a measuring error exceeding a given error limit.
In this second embodiment it is also possible to make provision that the measuring circuit actualizes the stored empty echo function from above to the level to be monitored by the limit sensor, irrespective of the result of the level comparison, every time the output signal of the limit sensor changes its status.